I was under the impression that since the reason for no cross region guesting is because of the split databases, we shouldn't be able to communicate at all with anyone from EU servers, let alone group with one.

This was what happened: -
Me and my buddy were LFM'ing on mapchat.
A german guy joined us.
We got error when trying to enter dungeon, so we asked him which region he was from and he said EU.

How the heck did he even get to know we were looking for more on mapchat?